Method for predicting and controlling the castability of liquid steel

ABSTRACT

The invention relates to a method for predicting and controlling the castability of liquid steel. According to said method, the chemical composition of a melt to be cast is analyzed, an alloy calculation is carried out, alloy elements and/or additions are determined in order to obtain defined material characteristics of the steel, and operating diagrams are drawn up for further treatment of the melt. The interactions of the alloy and/or addition elements, which influence the castability, are taken into account during the alloy calculation as supplemental conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2004/008300, filed Jul. 23, 2004 and claims the benefitthereof. The International Application claims the benefits of GermanPatent application No. DE10339595.4 filed Aug. 26, 2003. All of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a method for predicting and controlling thecastability of liquid steel by analyzing the chemical composition of amelt to be cast, carrying out an alloy calculation and defining alloyingelements and/or additives for obtaining specific material properties ofthe steel, and determining operating diagrams for further treatment ofthe melt.

BACKGROUND OF THE INVENTION

Such methods are used in the manufacture of steel. The liquid steel issupplied by a steelworks. The secondary metallurgy is then performed ina ladle furnace arranged upstream of a thin-strip continuous castingmachine. In addition, certain secondary metallurgical equipment isprovided in/on the ladle furnace for metallurgical treatment of theliquid steel. This equipment can be used to perform precise analyses ofthe melt and also for precise thermal conditioning of the melt. Theliquid steel is treated in the ladle furnace by adding alloying agents,slag formers, reduction agents, desulfurization agents, etc., wherethese additives are added automatically or manually. In addition, theslag can be treated by adding oxygen or by rinsing with an inert gassuch as argon. The liquid steel can be stirred electromagnetically inthe ladle, and electrical energy can also be supplied to it via carbonelectrodes. The arc passing from the electrodes to the melt causes thealloying elements to melt and enables the thermal conditioning of themelt.

In order to manufacture from the melt a specific steel quality havingdefined material properties such as strength, toughness, hardness,corrosion resistance etc., it is necessary to add metal and non-metalalloying elements and additives. Mathematical models are used for thispurpose, said models calculating from a latest analysis of the melt thematerial composition of the required alloying elements and additives inorder to obtain a very specific steel quality. The proportions of themetal and non-metal elements are thereby set in a defined band.Additional strength formulae that take account of the interactionsbetween the alloying elements and additives in the melt are applied in aquality center in order to assess the expected material properties.These formulae are mainly empirical. In conventional works comprisingsteelworks, ladle furnace and continuous casting machine, suchcalculations of the interactions of the additives and alloying elementsare at best performed offline in quality centers. The strength formulaeand empirical formulae cited in the literature are simplified models forthe complex interactions of the alloying elements and additions thatinfluence the material properties of the cast steel.

Thin-strip continuous casting machines mainly manufacture steel stripshaving a strip thickness of up to 10 mm. The melt is conditionedfollowing an analysis in a similar way to in conventional machines. Ithas been found, however, that in thin-strip continuous casting machines,the castability of the liquid steel is far more problematic than inconventional casting machines, e.g. in continuous casting machines forslabs.

Liquid steel is designated as uncastable if the cast strip cracks e.g.when casting in the thin-strip continuous casting machine, the castmaterial exhibits surface defects or structural faults of a generalnature and it causes plant breakdowns as a result of the uncastableliquid steel e.g. sticking to the casting rollers etc. Until now,attempts to solve these problems with the castability have largely beenmade in the thin-strip continuous casting machine itself. These attemptswere only partially successful, however, because many melts proveduncastable.

SUMMARY OF THE INVENTION

The invention is therefore based on the problem of improving the methodfor predicting and controlling the castability of liquid steel so as tosignificantly reduce the failure rate.

In order to solve this problem, in a method of the type specified in theintroduction it is provided according to the invention that theinteractions of the alloying elements and/or additive elementsinfluencing the castability are taken into account in the alloycalculation as supplementary conditions.

The invention is based on the surprising discovery that interactionsbetween the alloying elements and/or additive elements exist that arenot only relevant to the mechanical properties but also to thecastability of the melt. These are new and different interactions thatare independent of the known interactions taken into account up to now.According to the invention, the usual interactions must be taken intoaccount in the alloy calculation as before, i.e. the proportions of theindividual alloying elements and additives must lie within defined validranges. In addition, the supplementary conditions, which are derivedfrom the interactions and influence the castability, must be taken intoaccount.

In the method according to the invention it has proved particularlyadvantageous if at least two alloying elements and/or additives at atime are related to each other to determine the effect of theirproportions on the castability. Two materials at a time are related toeach other in an x-y coordinate system on the basis of data gatheredfrom melts already cast. The relative proportions of the melts, forexample in percent or in PPM, are plotted along the axes. In addition, atolerance band specifying the minimum value and the maximum value foreach element is specified for each alloying element and/or each additivein the form of straight lines parallel to the axes. When theinteractions are not taken into account, a rectangular intersecting setdefined by the intersecting straight-line segments is obtained. Inaddition, the current proportions of the two alloying elementsrepresented are plotted in the coordinates system, this instantaneousvalue being symbolized by a point. It is then immediately obvious in thegraph whether the melt does or does not lie within the tolerance band.In order to obtain a castable melt, however, it is not enough for themelt to lie within the permitted range. According to the invention,those interactions that influence the castability of the melt must alsobe taken into account. In order to be able to take into account the datagathered on the melts, it is provided in the method according to theinvention that the information “castable” or “uncastable” is assigned toeach cast melt.

Using this information, it is provided in the method according to theinvention that, based on the data gathered on cast melts and based onthe alloying elements and/or additives related to each other, at leastone permitted range, within which a castable melt is expected, isdefined for the proportions of alloying elements and/or additives. Thispermitted range is a subset of the aforementioned range for which onlythose interactions that influence the material properties are taken intoaccount. It has proved, however, that the first, larger range cannot befully used because, owing to the interactions that influence thecastability, problems arise in many cases so that the melt isuncastable. Hence, the range that specifies the permitted proportions ofthe individual alloying elements and additives must be adjusted to takeinto account the data gathered on melts already cast, i.e. must bereduced in size. If the gathered data is taken into account, with eachmelt being assigned the information “castable” or “uncastable”, then itis possible to define specific ranges as permitted in which those meltslie that have proved castable in the past.

In order to minimize the computing overhead, it can be provided in themethod according to the invention that the permitted range for theproportions is defined as an intersecting set of a plurality ofinequalities. The whole x-y surface of the coordinate system can bedivided into two sections by an inequality, namely into a valid and aninvalid region. An area on one side of a straight line is the graphicalequivalent of an inequality. In addition, the coordinate axes can beused to define permitted ranges, because, since the alloying elementscan only ever assume positive values, only the first quadrant need beconsidered.

In the method according to the invention, it will be necessary ingeneral to define the permitted range as an intersecting set of aplurality of intersecting straight lines. Assuming the axes of thecoordinate system are not taken into account, at least three straightlines are required in order to define the range uniquely. In practice ithas proved that more than three, in particular four, inequalities areoften required for a sensible definition of the range.

The method according to the invention can be implemented particularlyquickly and in part automatically if the interactions of the alloyingelements and/or additive elements are implemented as mathematical modelsin a computer system. The calculation and graphical representation ofthe ranges require comparatively little computing time, so that it canbe established immediately after performing a melt analysis whether theproportions of the individual alloying elements and additives lie in thepermitted range or whether additional treatment steps are required.According to a further embodiment of the invention, it can also beprovided that the method according to the invention for predicting andcontrolling the castability of liquid steel is performed automaticallyby the computer system iteratively.

It can also be provided in the method according to the invention thatfuzzy logic methods are used for the mathematical models. Alternativelyor additionally it can be provided that neural networks are used for themathematical models.

In order to keep the computation overhead for implementing the methodwithin bounds, it can be provided that a preselection of those alloyingelements and/or additive elements that influence the castability of themelt is made for the alloy calculation. Studies have shown that onlysome of the alloying elements influence the castability. If a meltcomprises ten elements, it would be necessary to investigate the firstelement with the remaining nine elements, the second element with eightelements, etc., so that a large number of element pairs would need to betaken into account. It is therefore practical to take into account onlythose alloying elements and/or pairs of alloying elements and/oradditive elements that actually influence the castability of the melt.The number of element pairs to be taken into account can be reducedsignificantly in this way. This also reduces the number of inequalitiesi.e. boundary conditions to be taken into account, which simplifies thesolution of the equation systems.

According to one version of the method according to the invention, itcan be provided that interactions between the following alloyingelements and/or additives are taken into account in the alloycalculation: C, Si, Mn, S, Al, N, Zn, O₂. It has been found that therestriction to these eight alloying elements and/or additives issufficient to achieve a considerable improvement in the castability.

The method according to the invention can be embodied in such a way thatinteractions of the following pairs of alloying elements and/oradditives are taken into account in the alloy calculation: N/O₂, Zn/O₂,S/Zn, C/Zn, Mn/S, Mn/N, Si/C, Al/C, in particular Si/O₂, S/O₂, Al/O₂,S/C, N/C. In theory, 28 pairs can be combined from the eight selectedalloying elements cited. It has proved, however, that only 13 of thesepairs influence the castability. Of these, five pairs of alloyingelements or additives have a serious effect on the castability. Even ifonly these five pairs are taken into account with a view to achieving anefficient method it is still possible to achieve excellent results asregards predicting and controlling the castability.

In the method according to the invention, it can be provided that thepermitted range for one or each alloying element and/or one or eachadditive that results in a castable melt and the actual value measuredin the melt are shown on the same graph. The actual value can be plottedin the graph as a point or a cross or the like, so that an operator cansee at a glance whether it does or does not lie within the permittedrange. This graph is shown for each of the value pairs to be taken intoaccount, so that an operator identifies whether all the boundaryconditions that influence the castability are satisfied, or else heidentifies for which alloying elements further treatment is requirede.g. by adding more of the alloying element. In addition, it can beprovided that the permitted range for an alloying element and/or anadditive resulting from the desired material properties is shown.

It is useful if in the method according to the invention an updatedactual value of an alloying element or an additive is shown after atreatment step carried out on the melt. This makes it immediatelypossible to check whether the treatment step has led to the desiredsuccess.

Likewise, it can be provided that after a plurality of treatment stepscarried out on the melt, the respective actual values of an alloyingelement or an additive are shown as points connected together bystraight-line segments.

The method according to the invention can be used particularlyadvantageously in a thin-strip continuous casting machine operating onthe principle of the twin-roller casting process.

In addition, the invention relates to a control device for a secondarymetallurgical machine, in particular a ladle furnace, having a means foranalyzing the chemical composition of a melt to be cast, a means forcarrying out an alloy calculation to define alloying elements and/oradditives in order to obtain specific material properties of the steel,and means for determining operating diagrams for further treatment ofthe melt.

According to the invention, the control device is embodied to carry outthe method described.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and details of the invention are described using anexemplary embodiment with reference to the drawings, in which:

FIG. 1 shows the flow diagram for the method according to the invention;

FIG. 2 shows a graph of the ranges of two alloying elements where theinteractions are taken into account;

FIG. 3 shows a diagram showing the proportions of the elements sulfurand carbon dioxide, and the castable range; and

FIG. 4 shows a diagram showing the proportions of the elements siliconand oxygen, and the castable range.

DETAILED DESCRIPTION OF THE INVENTION

The diagram shown in FIG. 1 shows the flow diagram of the method forpredicting and controlling the castability of liquid steel.

The method starts with a melt analysis to determine the chemicalcomposition of the melt to be cast. For the melt analysis, the melt isheld in a ladle furnace arranged upstream of a thin-strip continuouscasting machine. The metallurgical treatment of the liquid steel forsetting the required material parameters is carried out in the ladlefurnace. Electrical and/or thermal energy can be supplied to the meltvia graphite electrodes in order to trigger specific chemical reactions.The melt can be stirred electromagnetically in the ladle furnace.Alloying elements and additives such as slag formers, reduction agents,desulfurization agents etc. are added automatically or manually. Inaddition, there is the facility to rinse the liquid steel with an inertgas such as argon or to supply oxygen.

After carrying out the melt analysis, an alloy calculation 1 isperformed in order to set metal and non-metal alloying elements in adefined band. The alloy calculation is used to calculate the type andquantity of the additives and alloying elements in order to be able tomodify and treat the batch of liquid steel currently in the ladlefurnace so that it meets the requirements. First, the individualalloying elements must be present in the correct proportion i.e. in thecorrect concentration, where tolerance bands having a lower and upperlimit exist for each element. In this method, interactions between thealloying elements and additives that influence the castability areadditionally taken into account. The mathematical models used in thealloy equation 1 take into account these interactions, so that the meltis extremely likely to be castable after the treatment.

From past experience it was found that when using conventional methods,although a melt did satisfy the requirement regarding the materialproperties of the finished steel, surface defects still occurred or thesteel adhered to the casting rollers, for example, so that the melt hadto be rejected as uncastable.

Information about the castability is available after performing thealloy calculation 1. If it was calculated that the melt is castable, themethod continues with the determination of operating diagrams 2 for theelectric-arc furnace tapping and the ladle furnace. If the result of thealloy equation 1 is “uncastable”, more alloying elements or additives,for example, must be added, or treatment steps such as the addition ofan inert gas or oxygen are required. Depending on the prediction as tothe castability of the liquid steel, the operating diagrams foroperating the ladle furnace are determined, and stipulations are made asto the addition of metal and non-metal additives and the furthertreatment. The inclusion of the interactions that influence thecastability results in supplementary conditions for the operatingdiagrams or to a change in an operating diagram. The operating diagramsare determined for the ladle furnace and the secondary metallurgy.

This procedure is advantageous with regard to selecting the melts. If amelt proves to be uncastable, or if the measures to produce thecastability are too complex, then a decision can be made to reject themelt. In this case, the melt would need to be processed again in thesteel works. This procedure avoids costs of mistakes made in productionand saves resources.

If it is identified that the melt can be brought into a castable stateby metal or non-metal additives, and if this procedure is not toocomplex, then the castability of the melt can be achieved using thedetermined operating diagrams for the ladle furnace and the secondarymetallurgy. In this case as well there is the advantage of avoidingcosts of mistakes in production and saving resources.

If it is identified that the melt is castable, the melt can be treatedin the ladle furnace using the operating diagram intended for it, andcan be released for the thin-strip continuous casting machine.

If it is identified that the melt can be set even more beneficiallymetallurgically by taking into account the interactions influencing thematerial properties, this can be done while maintaining the castability.This has the resultant advantage that the melt can be set optimallymetallurgically taking into account the castability. Once again in thiscase, costs of mistakes in production are avoided and resourcesprotected.

As can be seen in FIG. 1, control parameters for the further treatmentof the melt are obtained through the determination of the operatingdiagrams 2.

FIG. 2 shows a graph of the ranges of two alloying elements where theinteractions that influence the castability are taken into account.

The ranges for the proportions of the elements x and y respectively areplotted along the x-axis and y-axis respectively. The range of eachelement is limited by two straight lines parallel to the axes, whichdefine the minimum and maximum concentration respectively of the givenmaterial in the melt. If the analysis value of the melt lies within theintersecting set of these straight lines, the requirements for thematerial properties are satisfied.

It is not sufficient, however, merely to take into account therequirements for the material properties. In addition, the interactionsthat influence the castability must be considered. In FIG. 2, thecastable region is shown by the straight-line segments 3, 4, 5. A meltthat not only satisfies the material properties requirements but alsothe castability requirements must lie in the intersecting set of bothareas. This valid region 6 is shown hatched in FIG. 2.

From a melt analysis, a value 7 is known that satisfies the conditionsfor the conventional casting operation as far as the material propertiesare concerned, because it lies within the range of the elements x and y.It does not lie within the valid region 6, however, so it is likely thatthis melt will be uncastable.

The analysis of another melt has produced the value 8 that lies withinthe valid area 6. This means that not only are the material propertiesrequirements satisfied, because the two elements x and y lie within therespective tolerance ranges, but also the castability is establishedbecause the value 8 lies within the straight-line segments 3, 4, 5. Asfar as these elements x and y are concerned, the melt can be cast.

This investigation, explained using the elements x and y by way ofexample, must be carried out for all relevant value pairs, all of whichmust lie within the valid range. The following value pairs must beinvestigated as a minimum: Si/O₂, S/O₂, Al/O₂, S/C, N/C. If theinvestigation of these conditions leads to the result that all theelements lie within the valid ranges, it is highly likely that the meltis castable. If any value does not lie within the valid range, anothertreatment step is required, for example by adding an alloying element.It must be realized, however, that the proportions or, as the case maybe, concentrations of the other alloying elements and additives to betaken into account are also affected by the addition of an alloyingelement. These interrelationships are generally non-linear and complex.The mathematical models that take into account these interactions thatinfluence the castability therefore include methods such as neuralnetworks or fuzzy logic. Hence in general an iterative calculation, i.e.an optimization calculation is carried out in order to achieve theobjective using the minimum quantity of alloying elements and/or atminimum cost.

FIG. 3 shows a diagram of the proportions of the elements sulfur andcarbon. The concentration of carbon in the melt is plotted along thex-axis, and the concentration of sulfur along the y-axis. The triangulararea 9 shown in FIG. 3 shows the region of castability of the elementpair sulfur/carbon. The first analysis value 10 lies outside thetriangular area 9 i.e. the melt is not castable in this state. Hence atreatment of the melt is carried out, for instance by adding an additivein order to increase the proportion of carbon and reduce the proportionof sulfur. An analysis is carried out again after this treatment,resulting in the analysis value 11. Although the proportions of thesetwo elements now lie near the region of castability, a second treatmentstep is still necessary until the analysis value 12 is obtained. Theanalysis value 12 lies within the triangular area 9, i.e. within thecastable region. It must be ensured at the same time, however, that theother elements and/or pairs of elements to be taken into account liewithin their valid regions.

FIG. 4 shows the proportions of the elements silicon and oxygen and thecastable region.

Unlike in FIG. 3, the castable region 13 in FIG. 4 is defined by aplurality of straight-line segments that do not form an enclosed area.In some cases, the castable region can also be defined by parabolasegments or segments of trigonometric functions. The aim should,however, be to define the valid regions by straight-line segments inorder to keep the computing overhead within bounds.

The first analysis value 14 lies outside the castable region 13. Aftertreatment of the melt the analysis value 15 was obtained, in whichalthough the oxygen content was increased, it was too high, with theresult that the value 15 again lay outside the castable region 13. Theanalysis value 16 that meets the conditions for the elements silicon andoxygen was only measured after another treatment step.

In the method it is provided that the diagrams for the five mostimportant pairs of elements are displayed simultaneously to the operatoron a display. In addition, the analysis values for individual alloyingelements or additives can be displayed as numerical values in a table.By this means the operator can see at a glance which values are alreadyacceptable and which values require further treatment.

After each melt the measured values of said melt are saved in a databaseso that the mathematical models can have recourse to a constantlygrowing database, thereby increasing the prediction probability.

1. A method for controlling the castability of liquid steel, the methodcomprising: selecting each pair of alloying elements from the groupconsisting of Si/O₂, S/O₂, Al/O₂, S/C, and N/C; for the each pair ofalloying elements from the group consisting of Si/O₂, S/O₂, Al/O₂, S/C,and N/C, establishing a first range of relative concentration limits ina melt such that a subsequent casting of the melt is likely to exhibitacceptable mechanical properties; for the each pair of alloying elementsfrom the group consisting of Si/O₂, S/O₂, Al/O₂, S/C, and N/C,establishing a respective second range of relative concentration limitsas a subset of the first range of relative concentration limits suchthat a subsequent casting of the melt is further likely to be castable;and casting a steel melt comprising the each pair of alloying elementsfrom the group consisting of Si/O₂, S/O₂, Al/O₂, S/C, and N/C havingrelative concentration limits within the second range of relativeconcentration limits for the each pair of alloying elements.
 2. Themethod of claim 1, further comprising: for the each pair of alloyingelements from the group consisting of Si/O₂, S/O₂, Al/O₂, S/C, and N/C,displaying the first range on a graph illustrating concentration of afirst element along a first axis and concentrations of a respectivesecond element along a second axis; for the each pair of alloyingelements from the group consisting of Si/O₂, S/O₂, Al/O₂, S/C, and N/C,displaying the respective second range on the graph as a sub-area of thefirst range; and for the each pair of alloying elements from the groupconsisting of Si/O₂, S/O₂, Al/O₂, S/C, and N/C, displaying a measuredrelative concentration of the first element and the respective secondelement in the melt as a point on the graph.
 3. The method of claim 1,used in a thin-strip continuous casting machine according to a twinroller casting process.
 4. The method of claim 1, further comprisingtreating the steel melt by increasing an amount of a first element in afirst pair from the group consisting of Si/O₂, S/O₂, Al/O₂, S/C, and N/Cif a measured relative concentration of the first pair in the melt fallsoutside the respective second range.